Technical Abstract:
The objective of this study was to test the effect of age of the ovulatory follicle on fertility in beef cows. Ovulation was synchronized with the 5 d CO-Synch + CIDR program in multiparous (n = 171) and primiparous (n = 130) postpartum beef cows in two groups (G1 and G2) before application of treatments. Cows in G1 received estradiol benzoate (EB; 1mg/500kg BW, i.m.) 5.5 d (n = 162) and G2 received a similar dose of EB 6.5 d (n = 139) after the final GnRH of the synchronization program to create follicular turnover. Within group, PGF (25 mg, i.m.) was administered either 5.5 d (“young” follicle, YF; n = 155) or 9.5 d (“mature” follicle, MF; n = 146) after EB. In the MF treatment, estrous detection and AI were performed for 3 d after PGF, and timed-AI (TAI), coupled with GnRH administration, was performed at 72 h after PGF for cows not detected in estrus. In the YF treatment, estrous detection was performed for 4 d, with TAI at 96 h after PGF if estrus was not detected. Ovarian ultrasonography was performed in YF and MF at EB, PGF and AI, and 5.5 d after EB (MF only). Cows that failed to initiate a new follicular wave after EB (G1, n = 6; G2, n = 5) were excluded from further analyses. Also, cows in the MF treatment that initiated a second follicular wave after EB, but before PGF (G1, n = 25; G2, n = 22) were excluded from further analyses. Within the first 72 h after PGF, more MF cows (76.6%) than YF cows (48.3%; P < 0.01) exhibited estrus. Throughout the estrous detection period, proportion detected in estrus and interval from PGF to estrus were greater (P < 0.01) in the YF than MF treatment (88.6 vs. 76.6%, 79.0 ± 0.7 vs 56.7 ± 1.7 h, respectively). Diameter of the ovulatory follicle was greater (P < 0.01) with estrus-AI (13.3 ± 0.1mm) than TAI (12.6 ± 0.2mm) but did not differ between treatments (MF, 13.1 ± 0.2 mm; YF, 13.0 ± 0.1 mm). Pregnancy rate in the MF (72.3%) and YF (67.1%) treatments did not differ, however, pregnancy rates in estrus-AI (75.1%) was greater (P < 0.01) than in TAI (55.4%). In summary, age of the ovulatory follicle resulted in a longer interval to estrus and to AI in cows with young follicles but did not influence pregnancy rate.